This is a slightly revised version of an article (updated 3 April 2009) which first appeared in the February 1999 edition of
The Cartridge Researcher, the
official bulletin of the European Cartridge Research Association. Additional information provided by Bob Mackenzie.

The Second World War saw a major change in the
nature of infantry anti-tank weapons. In 1939, most armies fielded
large, high-velocity rifles in calibres varying from 7.92mm to 20mm. By
1945 these were virtually obsolete, replaced by much lower-velocity
weapons firing large-diameter shaped-charge warheads capable of cutting
through the thickest armour on the battlefield. While the British PIAT
(Projector, Infantry, Anti-Tank) was a spigot mortar, two other types
of weapon - the rocket and the recoilless gun - proved to be more
successful bases for development and their successors are still in
service today. In the middle of the war a strange hybrid appeared, from
a nation not even involved in the fighting, which combined features
from both eras; the Carl Gustav m/42.

The problem of providing infantrymen with some
defence against the tank appeared shortly after the tank itself. While
the Germans discovered that the early British tanks were sufficiently
slow and thinly-armoured to be vulnerable to direct artillery fire, it
has always been an important principle of morale that the ordinary
footsoldier should have some means of defending himself against the
enemy's battlefield weapons, so the anti-tank rifle (ATR) was born.

The first of the breed was the Mauser M1918, a
huge bolt-action rifle based around a big 13mm cartridge, firing a
bullet about four times heavier than the standard 7.92mm rifle at a
similar muzzle velocity. This was capable of penetrating around 20mm of
armour at 100 metres and 15mm at 300 metres, when striking at 90
degrees. As the early tanks carried no more than 12mm of armour, this
was an effective weapon.

Between the wars, many nations learned from the
German experience and developed similar rifles. The British adopted the
Boys rifle, virtually identical in principle but firing a slightly more
powerful .55" (13.9mm) cartridge, with a performance only marginally
better than the Mauser. The Soviets first tried a 12.7mm weapon which
was a direct copy of the Mauser, then switched to a much more effective
pair of guns - the PTRS and PTRD - designed around a massive new 14.5mm
round. In the search for more performance without excessive size and
weight, the Germans and Poles independently developed rifles in the
standard military calibre of 7.92mm, but using huge cartridge cases to
generate muzzle velocities of around 1,200m/s. Other nations took a
less subtle route and chose massive 20mm cannon to achieve the
necessary performance.

There were two problems with anti-tank rifles; as
tank armour rapidly thickened in the light of war experience, even the
best of them proved inadequate; and attempts to increase performance
led to huge, unwieldy weapons which needed a team to carry them. The
best of the rifles - the high-velocity 14.5mm Soviet guns and the
biggest of the 20mm cannon - were capable of penetrating up to 40mm of
armour plate at short range, but this performance fell off rapidly with
distance and even more as the striking angle departed from the
vertical. In 1939, few tanks had more than 30mm of armour but double
this was typical by the middle of the war, with three or four times as
much by 1945.

Then there was the problem of portability. The
Mauser Ml918 weighed 17.3kg and was 1.68m long. The Boys rifle was
fractionally shorter and lighter and the slim 7.9mm Polish Maroszek
weighed only 9kg. These, however, were all marginal even at the
beginning of the war. The much more effective Soviet PTRD probably
achieved the best compromise between performance and portability,
weighing only 16kg despite being nearly 2 metres long. Pity the poor
infantryman firing it, however; the muzzle energy was almost double
that of the Boys rifle!

The big 20mm guns really pushed portability to
the limit. There were some quite compact ones using low-velocity
cartridges, but the most effective were the Solothurn Sl8-l000 and the
Lahti L39, both based around the 20x138B round used in the German light
FlaK cannon, and the Japanese Type 97 which used a 20x125mm cartridge.
These weighed around 40-50kg (with the Type 97 tipping the scales at
68kg with its protective shield attached), but even these monsters were
not much use by the middle of the war.

Fortunately for the poor infantryman, various
nations were trying to make use of shaped charge warheads, in which an
explosive detonated around a metallic cone generates a narrow jet of
molten metal at extremely high velocity and with remarkable cutting
effect. It was clearly a useful principle against armour if only a
convenient method could be found of projecting it from a distance.

As already described, the British introduced the
PIAT, which fired a 1.36kg warhead capable of penetrating 10cm of
armour. It weighed 14.5kg and was only 99cm long, but suffered from
heavy recoil. The Americans introduced the bazooka, which avoided the
recoil problem by utilising a tube-fired rocket. The Germans developed
a range of recoilless guns, most notably the Panzerfaust shoulder-fired
weapon, as well as copying the bazooka principle. The counter-recoil
method of achieving recoilless firing had been known for decades and
the American Davis gun (which achieved this by firing an equal weight
of shot to the rear) had seen service in the Great War, but the Germans
perfected the method of diverting rearwards some of the high-velocity
gas from the propellant charge in order to balance the recoil. These
were all effective devices, limited principally by the low velocity
which gave them a steep trajectory and an effective range against
moving targets of around 100m.

The Swedish firm of Carl Gustav were also
experimenting with recoilless weapons but with a different aim; to
reduce the weight and recoil of a high-velocity, small-calibre weapon.
This work started in 1940 and resulted in the unique 20mm m/42. In
effect, this combined the old principle of defeating armour by means of
high-velocity solid shot with the latest technology to reduce size and
weight. It was designed around an enormous cartridge with a case some
180mm long, necessary because of the diversion of much of the
propellant to provide rearwards thrust via a blowout base to the
cartridge case. It was a single-shot weapon, manually loaded via a
simple lever-operated side-hinged breech containing a single exhaust
cone, and the gun was designed to rest on the firer's shoulder to
ensure that the back-blast did not injure him. The gas exited the gun
via a hole in a plate which formed a part of the breechblock. This was
gradually eroded by gas pressure and had to be replaced after
each twenty AP rounds were fired, and after each forty HE and practice
rounds (the AP rounds were loaded to a higher pressure); the state of
wear of this plate affected the muzzle velocity. The sights were
calibrated for 300m.

A 1943 assessment of factory drawings by no less
than Dennis Burney of the Broadway Trust Company (designer of the
recoilless Burney guns) estimated a muzzle velocity of 800-820m/s, but
it appears that he was assuming a heavier projectile, as most 20mm AT
weapons had bullet weights of 130-150g. In fact, the m/42's projectile
weight was 108g and the muzzle velocity around 950m/s. The AP shot had
a tracer and was known as the slpprj m/42; unusually for an ATR, there
was also an impact-fuzed HE projectile known as the sgr m/43.

Burney had some interesting technical comments to
make, comparing the design with his own very similar 3.45" recoilless
gun. He estimated that the size and shape of the exhaust jet in the
breech would limit the recoil absorption to about 80% but that it was
capable of improvement and was basically a satisfactory design.
Interestingly, he suggests that its main use could be as a sniping
weapon.

The gun was clearly capable of matching the
armour penetration of the best of the existing anti-tank rifles, yet it
was far smaller and lighter at only 1.4m long and 11.2kg.
Unfortunately, by 1942 this performance was no longer sufficient to be
worthwhile. Despite this, orders for 3,219 ATRs were placed, with
deliveries between August 1942 and July 1944. The first 500 were
faulty, and used only for training until they were repaired. By the end
of the war only 1,000 had been delivered.

There can be no doubt that the main value of the
m/42 was to provide experience which led the firm to the development of
the famous 84mm Carl Gustav recoilless gun, first introduced in 1946
and still in widespread use today. A range of ammunition types is
available and the handy 14.5kg gun has even seen action in British
hands as an anti-ship weapon - in South Georgia in 1981.

A postscript to this story is that 20mm Anti-Materiel Rifles
have recently been making a come-back. Two of them are South African, the NTW 20
(available in both the 20x82 MG151 and 20x110 Hispano calibres) and the Truvelo
(20x110). In addition, Croatia makes the RT20 (20x110), while Slovenia has
offered the Alpimex (20x82) and Finland the Helenius in 20x99R ShVAK.
However, all of these are bigger and heavier than the m/42 and, despite a token
attempt at recoil-reduction in the RT20, are definitely not recoilless!

Illustrations of the Carl Gustav m/42 with its ammunition in the Pattern Room: